lamda分组表达式
1 // 单条件分组，
2 Map<String, List<YourBean>> mapByOne = yourBeanList.stream().collect(Collectors.groupingBy(YourBean::getBillDateStr));
3
4 // 分组后，统计每组中数据量
5 Map<String, Long> count = yourBeanList.stream().collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.counting()));
6
7 // 分组后，求出每组中某属性的平均值
8 Map<String, Double> avg = yourBeanList.stream().filter(i -> i.getGoodAmount() != null).
9 collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.averagingDouble(YourBean::getGoodAmount)));
10
11 // 分组，某属性求和
12 Map<String, Double> sum = yourBeanList.stream().filter(i -> i.getGoodAmount() != null).
13 collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.summingDouble(YourBean::getGoodAmount)));
14
15 //对求和的结果集进行从大到小排序
16 Map<String, Double> finalMap = new LinkedHashMap<>();
17 sum.entrySet().stream().sorted(Map.Entry.<String, Double>comparingByValue().reversed()).forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));
18
19 // 分组后，通过join组成新的map
20 Map<String, String> joinNewMap = yourBeanList.stream().filter(i -> i.getGoodAmount() != null)
21 .collect(Collectors.groupingBy(YourBean::getBillDateStr,
22 Collectors.mapping(i -> i.getGoodAmount().toString(), Collectors.joining(", ", "Post titles: [", "]"))));
23 // 2022-04-23
24 // Post titles: [3.0, 1.0, 2.0, 3.0]
25
26 // 转换分组结果List -> Set
27 Map<String, Set<String>> namesByName = yourBeanList.stream()
28 .collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.mapping(YourBean::getGoodName, Collectors.toSet())));
29 Set<String> x = namesByCity.keySet();
30
31 // 两个条件分组
32 Map<String, Map<String, List<YourBean>>> mapByTwo = yourBeanList.stream()
33 .collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.groupingBy(YourBean::getGoodName)));
34
35 // 使用java8 stream groupingBy 操作,按日期分组list,将List转化为name的List
36 Map<String, List<String>> mapList = yourBeanList.stream()
37 .collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.mapping(YourBean::getGoodName, Collectors.toList())));
